In the continuous casting of molten steel a break ring is used to provide a thermal barrier at the interface between the furnace nozzle and the mold. The break ring must possess thermal shock resistance, stability at high temperature and must be corrosion and erosion resistant to the high temperature molten steel which flows through the break ring into the mold. The ability to resist attack from the molten metal passing through the break ring as it flows into the extrusion mold determines the tonnage of metal that can be cast in a single operation without interruption of the process to replace the break ring. Cost is another important factor which requires the break ring to be machinable.
Boron nitride is a conventional material used in the fabrication of break rings. It is desirable because of its good thermal shock resistance, stability at high temperature and machinability. However, it lacks good abrasion resistance which subjects it to high wear rates when exposed to flowing molten metal. Boron nitride has also been combined to form a ceramic composite with alumina (Al.sub.2 O.sub.3) which is also used in molten metal applications due to its hardness, abrasion resistance and chemical stability. In addition, boron nitride has been separately combined with aluminum nitride (AlN), titanium diboride(TiB.sub.2), mullite(3Al.sub.2 O.sub.3 --2SiO.sub.2) and with aluminum nitride and titanium diboride. Other materials have also been combined with boron nitride such as silicon nitride to form a composite for use as a break ring. However a silicon nitride composite is not readily machinable. The boron nitride composites BN--AlN, BN--3Al.sub.2 O.sub.3 --2SiO.sub.2, and BN--TiB.sub.2 --AlN are readily machinable and are commercially available from the Praxair Inc., advanced ceramics division, located in Cleveland, Ohio. The mechanical and physical properties of Al.sub.2 O.sub.3 --BN and mullite-BN composites are described in Lewis et al in "Microstructure and Thermomechanical Properties in Alumina and Mullite Boron Nitride Particulate Ceramic-Ceramic Composites", Ceram. Eng. Sci. Proc. 2: 719-727 (Nos.7-8,1981) which also includes data on the thermal shock resistance of such composites. In addition, U.S. Pat. No. 4,997,605 discloses a hot pressed ceramic composite formed from a blend of fused zirconia mullite and boron nitride which is indicated as having good resistance to thermal shock and reasonably good erosion and corrosion resistance to metal alloys.
The above identified ceramic composites of boron nitride which are readily machinable are all currently formed by hot pressing and have substantially similar corrosion and erosion resistant properties under test conditions which simulate the process conditions of a continuous casting operation. The composite formulation of any of the known boron nitride composites may be adjusted to increase its corrosion and erosion resistance but only as a tradeoff against other properties particularly machinability.